Measuring and Evaluating Computer System Performance

Transcription

1 Measuring and Evaluating Computer System Performance

2 Performance Marches On... But what is performance?

3 The bottom line: Performance Car Time to Bay Area Speed Passengers Throughput (pmph) Ferrari 3.1 hours 160 mph Greyhound 7.7 hours 65 mph Time to do the task execution time, response time, latency Tasks per day, hour, week, sec, ns... throughput, bandwidth

4 How to measure Execution Time? % time program... program results u 12.9s 2:39 65% % Wall-clock time? user CPU time? user + kernel CPU time? Answer:

5 Our definition of Performance Performance X = 1 Execution Time X, for program X only has meaning in the context of a program or workload Not very intuitive as an absolute measure, but most of the time we re more interested in relative performance.

6 Relative Performance can be confusing A runs in 12 seconds B runs in 20 seconds A/B =.6, so A is 40% faster, or 1.4X faster, or B is 40% slower B/A = 1.67, so A is 67% faster, or 1.67X faster, or B is 67% slower needs a precise definition

7 Relative Performance, the Definition Relative Performance (X/Y) Performance X Performance Y Execution Time Y Execution Time X = = = n "X is n times faster than Y" "X is n times as fast as Y" "From Y to X, speedup is n"

8 Example Machine A runs program C in 9 seconds, Machine B runs the same program in 6 seconds. What is the speedup we see if we move to Machine B from Machine A? Machine B gets a new compiler, and can now run the program in 3 seconds.???

9 What is Time? CPU Execution Time = CPU clock cycles * Clock cycle time Every conventional processor has a clock with an associated clock cycle time or clock rate Every program runs in an integral number of clock cycles Cycle Time MHz = millions of cycles/second, GHz = billions of cycles/second X MHz = 1000/X nanoseconds cycle time Y GHz = 1/Y nanoseconds cycle time

10 How many clock cycles? Number of CPU cycles = Instructions executed * Average Clock Cycles per Instruction (CPI) Computer A runs program C in 3.6 billion cycles. Program C consists of 2 billion dynamic instructions. What is the CPI?

11 How many clock cycles? Number of CPU cycles = Instructions executed * Average Clock Cycles per Instruction (CPI) A computer is running a program with CPI = 2.0, and executes 24 million instructions, how long will it run?

12 All Together Now seconds CPU Execution Time Instruction CPI = Count X X Clock Cycle Time instructions cycles/instruction seconds/cycle

13 CPU Execution Time Instruction CPI = Count X X Clock Cycle Time IC = 1 billion, 500 MHz processor, execution time of 3 seconds. What is the CPI for this program? Suppose we reduce CPI to 1.2 (through an architectural improvement). What is the new execution time?

14 Who Affects Performance? CPU Execution Time Instruction CPI = Count X X Clock Cycle Time programmer compiler instruction-set architect machine architect hardware designer materials scientist/physicist/silicon engineer

15 Performance Variation CPU Execution Time Instruction CPI = Count X X Clock Cycle Time Number of instructions CPI Clock Cycle Time Same machine different programs same programs, different machines, same ISA Same programs, different machines

16 MIPS MFLOPS Other Performance Metrics

17 MIPS MIPS = Millions of Instructions Per Second = Instruction Count Execution Time * 10 6 = Clock rate CPI * 10 6 Program-independent? Deceptive

18 FLOPS FLOPS = FLoating-point Operations Per Second Program-independent? Which operations? Useful, sometimes "Theoretical peak" FLOPS, peak FLOPS, sustained FLOPs How does execution time depend on FLOPS?

19 Which Programs? peak throughput measures (simple programs)? synthetic benchmarks (whetstone, dhrystone,...)? "kernels" of useful computation (lapack, fftw,...) Real applications SPEC (best of both worlds, but with problems of their own) System Performance Evaluation Cooperative Provides a common set of real applications along with strict guidelines for how to run them. provides a relatively unbiased means to compare machines.

20 Danger in Benchmark-Specific Performance Measures measures compiler as much as architecture (what about kernels?)

21 SPEC Performance on Pentium III and Pentium 4

22 Amdahl s Law The impact of a performance improvement is limited by the percent of execution time affected by the improvement Execution time after improvement = Execution Time Affected Amount of Improvement + Execution Time Unaffected Make the common case fast!!

23 Key Points Be careful how you specify performance Execution time = instructions * CPI * cycle time Use real applications Use standards, if possible Make the common case fast